Among other significant operational parameters, flexibility to use diverse feed stocks is often critical to economic operation of coal and coke gasification plants. For example, depending on the specific market conditions, desirable feed stocks can range from low sulfur sub-bituminous coal to high sulfur bituminous coal or petroleum coke. As a consequence, the hydrogen sulfide concentration in the acid gas that is separated from the sour syngas in the acid gas removal unit often varies due to the varying sulfur content in different gasifier feedstocks. This acid gas is often directed to a Claus sulfur recovery unit, where hydrogen sulfide and other sulfur compounds in the gas are converted to saleable elemental sulfur. However, and especially when hydrogen sulfide concentration in the acid gas is below a critical level, Claus unit efficiency may drop to a critical level.
To improve Claus process efficiency for feed gases with relatively low sulfur content, Claus plants may be run with oxygen as the oxidant (instead of air). Most typically, feed gas to an oxygen-blown Claus plant should comprise at least 20-25 mol % of sulfur compounds to maintain economic operation. Where the feed gas to the Claus plant is even leaner, treated syngas (exiting the acid gas removal unit) or other support fuel must often be combusted in the acid gas burner/reactor to maintain proper temperature. Such an oxygen-blown Claus plant with support fuel can process feed gases with hydrogen sulfide concentrations as low as 10 mol %.
Where the sulfur concentration in the Claus feed gas drops below these critical levels (e.g., below 10-25 mol %), addition of oxygen and/or supplementation with support fuel often fails to remedy the above problems in a practical or economic manner. In such cases, selective acid gas removal treatment systems are employed in which hydrogen sulfide and carbon dioxide are sequentially removed from the sour syngas using relatively complex configurations and low temperature as described, for example, in U.S. Pat. No. 4,430,316. While such configurations tend to expand the usefulness of the Claus process in gasification plants with low-sulfur feedstocks, various difficulties remain. Among other things, such known selective acid gas removal configurations are often capital intensive, and require relatively large quantities of energy. Moreover, such configurations typically do not allow for flexibility and can only be used for a specific and narrow range of gasifier sour syngas sulfur concentrations.
Therefore, while numerous configurations and methods for Claus plant operation with sour feed gas streams containing variable sulfur content are known in the art, all or almost all of them suffer from one or more disadvantages. Consequently, there is still a need to provide improved configurations and methods for treating feed gases with variable and/or low sulfur content.